Road speed governing systems are generally known in the automotive and heavy duty truck industries. Such systems are typically computer controlled wherein a control computer is responsive to a present vehicle speed signal to limit engine fueling signals to thereby limit vehicle speed to a programmed maximum road speed limit. An example of a portion of one prior art engine and road speed governing control system is illustrated in FIG. 2.
Referring to FIG. 2, the prior art engine and road speed governing control system includes a control computer 5, typically referred to as an Engine Control Module (ECM), Engine Control Unit (ECU) or the like. Control computer 5 receives engine/vehicle operating signals from a number of sensors and other electrical components such as, for example, an accelerator pedal signal via signal path 24, an engine speed signal via signal path 32 and a vehicle speed signal via signal path 36. Control computer 5 is responsive to at least the foregoing signals to produce one or more fuel command signals, via signal path 58, for controlling a fuel system of an internal combustion engine.
Control computer 5 includes a fuel request calculation block 80 receiving the accelerator pedal signal via signal path 24, wherein block 80 is operable to compute an engine speed reference value ESREF therefrom and provide the ESREF value to a summing node .SIGMA.. The summing node .SIGMA. subtracts the ESREF signal from the engine speed signal supplied thereto via signal path 32, and produces a corresponding error signal which is supplied to an engine speed governor 82. Gov 82 may be any one of a variety of known engine speed governors such as a PID, all-speed or other known engine speed governor. Gov 82 produces an engine speed governor fuel command signal ESG FUEL, which corresponds to engine fueling conditions required to achieve the desired engine speed, and supplies the ESG fuel value to one input of a MIN block 84.
Control computer 5 further includes a road speed governor (RSG) 86 which includes a max road speed block 88 representing a programmed maximum road speed value, RSREF, stored within control computer 5. The maximum road speed value RSREF is supplied to a road speed limiter (RSL) 90 which also receives the vehicle speed signal supplied thereto via signal path 36. RSL 90 produces a road speed governor fuel command signal RSG FUEL, and supplies the RSG fuel value to a second input of MIN block 84. The output of MIN block 84 provides the fuel command signal, FUEL COMMAND, on signal path 58.
In operation, RSL 90 is operable to compare the vehicle speed signal with RSREF, and if vehicle speed is less than RSREF, RSL 90 produces a maximum fuel command as the RSG FUEL value on signal path 92. In this case, MIN block 84 is correspondingly operable to produce the ESG FUEL value on signal path 58 as the commanded fueling signal FUEL COMMAND. If, on the other hand, the vehicle speed signal exceeds RSREF, RSL 90 produces an RSL FUEL command as the RSG FUEL value on signal path 92, wherein the RSL FUEL command corresponds to engine fueling conditions in which actual vehicle speed is limited to the maximum road speed value RSREF. Since RSG FUEL will typically be less than ESG FUEL under such road speed limiting conditions, the FUEL COMMAND signal produced by MIN block 84 will accordingly be equal to RSG FUEL.
While road speed governing systems such as that just described have been extensively used in the automotive and heavy duty truck industries, they have certain drawbacks associated therewith. For example, while cruise control systems are often used by drivers as a technique for automatically limiting vehicle speed to a desired road speed, no such mechanism exists for limiting vehicle speed to a desired road speed under manual fueling conditions (typically via an accelerator pedal). As another example, fleet owners/managers typically set the maximum road speed value at or near a maximum speed limit likely to be encountered by the vehicle. Accordingly, passing another vehicle can be difficult, and sometimes dangerous, if vehicle speed is limited to the maximum road speed value during such passing attempts. This is particularly true when both vehicles involved in a passing maneuver have identically matched maximum road speed values, since the vehicles will actually be limited to different maximum road speed values due to mechanical differences therebetween (e.g. tire wear, etc.).
What is therefore needed is an improved road speed control system which overcomes the drawbacks associated with prior art road speed governing systems. Such an improved road speed control system should ideally allow the driver to adjust the maximum road speed value within some range of road speeds, and should further allow the driver to over-ride the maximum road speed value, at least temporarily, to facilitate the passing of another vehicle.